latest -git triggers an irqtrace/lockdep warning of a leaked
irqs-off condition:

  BUG: at kernel/fork.c:1033 copy_process()

after some debugging it turns out that commit ca1b940c accidentally left
interrupts disabled - which trickled down all the way to the first time
we fork a kernel thread and triggered the warning.

the fix is to re-enable interrupts in the 'else' branch of
setup_boot_APIC_clock()'s pmtimers calibration path.
Reported-by: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Thomas Gleixner <tglx@brown.paperbag.linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>